The present invention relates to a technique effectively applied to a high frequency power amplification circuit which is used for a wireless communication system such as a portable telephone, amplifies a high frequency signal, and outputs the amplified signal, and to an electric part in which the high frequency power amplification circuit is assembled. More particularly, the invention relates to a technique effectively used for reducing fluctuation of an output power and current consumption accompanying load fluctuation in a wireless communication system having a detection circuit for detecting the output level necessary for output power feedback control by a current detecting method.
Generally, a high frequency power amplification circuit for amplifying a modulated signal is assembled in a transmitting unit in a wireless communication system (mobile communication system) such as a portable telephone. In a conventional wireless communication system, to control the amplification factor of a high frequency power amplification circuit in accordance with a transmission request level from a baseband circuit or a control circuit such as a microprocessor, the output level of the high frequency power amplification circuit or an antenna is detected and fed back (refer to, for example, Japanese Unexamined Patent Publication No. 2000-151310). Hitherto, the output level is generally detected by using a coupler, a detector, or the like. The detector is often constructed as a semiconductor integrated circuit separate from the high frequency power amplification circuit.
The coupler is a device for detecting an output level via a capacitance formed between an output line (microstrip line) formed on a discrete part or an insulating substrate (module substrate) and a conductor disposed in parallel with the output line. The size of the coupler is larger than that of a device formed on a semiconductor chip. A directional coupler is described in, for example, “Basics and applications of microwaves”, Sogo Electronics Press, Jul. 10, 1997, pp. 191 to 193. Aceramic-stacked-layerlow-pass filter formobile communication and a directional coupler are described in “Electronic Materials”, Kogyo Chosakai, Publishing Co., Ltd., April issue, 1999, pp. 91 to 95.
Since a number of semiconductor integrated circuits and electronic parts are used separately from the high frequency power amplification circuit in the conventional method of detecting the output level of the high frequency power amplification circuit, it is difficult to reduce the size of the module. Further, in the case of using a coupler, a reference voltage may be applied to an end of the coupler in order to improve detection sensitivity. In this case, since it is necessary to optimally set the reference voltage and adjust a voltage or the like according to variations in parts, a problem occurs such that a burden on a set maker increases. When the coupler is used, a problem of power loss arises.
Further, as a portable telephone of recent years, a dual-band portable telephone is proposed, which can deal with a signal of, not only a system called GSM (Global System for Mobile Communication) using frequencies in the band of 880 to 915 MHz but also a system such as DCS (Digital Cellular System) using frequencies in the band of 1,710 to 1785 MHz. In a high frequency power amplification module used for such a portable telephone, an output power amplifier is also provided according to each band, so that a coupler for detecting the output level of the output power amplifier is also necessary for each of the bands. Therefore, it becomes more difficult to reduce the size of the module.
The applicant of the present invention consequently has achieved and applied an invention (Japanese Patent Application No. 2000-523757) of a wireless communication system of a current detecting method, which includes a transistor for output detection for receiving an input signal of a power amplification transistor for amplifying a high frequency signal and passing current proportional to current flowing in the power amplification transistor, and a current mirror circuit for transferring the current of the transistor. The current transferred from the current mirror circuit is converted to a voltage, and the voltage is used as an output level detection signal. The output level detection signal is compared with a transmission request level and, on the basis of the result, the output level is controlled.
[Patent document 1]
Japanese Unexamined Patent Publication No. 2000-151310